Organic waste
management by a smallscale innovative automated
system of anaerobic
digestion
Supported by the European Commission
under the research for SME associations theme
of the 7th Framework Programme for Research and
Technological Development
1st August 2012 to 31st July 2015
Energy management module
The energy management system is responsible for :
 biogas valorisation (small, certified biogas boiler)
 heating of the biological process (heat exchanger for
hot water bath)
COMBUSTION
HEAT RECOVERY
System requirements
1) Biogas valorisation
 CH4 concentration related to
the type of feedstock used and
digester operation
 Use of natural gas during startup, or transient digester
operation
 H2S content up to 2-3% by vol
 H2S levels should be reduced
below 50 ppm and biogas should
be dried
 water vapour, even more so due
to the higher operating
temperature
2) Heating of the biological process
“Thermophillic” mode requires an operating temperature of 57 oC ± 2 oC
3) Cost effectiveness and H&S approval
Adjust the mixture of air and fuel
4) Safe and User friendly system
Design answers
 12 kW natural gas boiler (to cope with start up)
 + 6 kW thermal output modified natural gas boiler running only on steady state
biogas stream (approximately 60/40 % vol of CH4/ CO2)
 small natural gas assisted flare to deal with poor quality intermittent biogas for
Environmental and H&S reasons
 Simple PLC, Relays and Switches for Automation (gas and water circuits)
 Detectors for CO, HC & Temperature Linked to Warning/Cut Off Systems
 HAZOP undertaken to maximise safety
Modified biogas boiler
• Burner head: cylindrical multi-hole premixed burner
• Hundreds of the identical holes producing premixed laminar lean flames
• The panel controls both of the flowrate of the incoming fuel and regulation of the
water circuits. Direct modification to this panel is difficult.
• Minimum modification should be considered for lower cost, H&S approval and
straightforward service by any gas engineer in the future
System front view
System back view
Components
•Biogas solenoid valve
•Biogas particulates filter
•Biogas spill valve to
control and maintain the
pressure
•Drain pots to collect
possible condensate in
the gas both before and
after the biogas blower
(to be checked and
emptied on a daily basis)
•Biogas blower
•Flare spill valve
•Natural Gas Assisted
Flare to burn off the
low quality biogas
produced during
transient periods
(Large Bunsen
Burner) for
environmental and
H&S reasons
•Blow off valve
•Blow off vent
•Secondary circuit drain
•Secondary circuit pump
•Temperature sensors show temperature after
biogas boiler and after natural gas boiler
•NG solenoid valve
•NG meter
•NG condensation drain
•Primary air bleeds
•Primary circuit pump
•Primary circuit drain
•Primary water over pressure vent
•Temperature sensor to monitor flow/return
•Heat exchanger (HE)
•3 way valve to control heat supply from HE
•Heat meter to record the amount of heat
supplied by the BB. It has an internal display
and a pulsed output available.
•Biogas boiler (BB)
•Natural gas boiler (NGB)
•Primary circuit expansion pressure tank +
fill point and pressure gauge on top
•Biogas pressure sensor
•Fire and heat sensor
•Carbon monoxide sensor
•Methane sensor
•LPG sensor (if used)
•Electricity meter
•Fire and gas panel
•Canopy ventilation fan
•Relays timers for the coordination of the
combustion process and subsequently hot
water production and regulation.
Components
Spill Valves for Flare, Emergency
Blow Off: biogas control. Heat
Energy Meter: records the
amount of heat supplied by the
biogas
Condensation Pots for Biogas
Moisture: To remove moisture in
the biogas in order to improve
the quality of combustion
including a filter for particulates.
Exterior connections:
physical connections between
the combustion module and
water feed, main support fuel,
biogas etc.
Sequence of Operation (Summary)
WATER LOOP
 Digester calls for Heat (thermocouple – settings can be changed)
 Heat is supplied by a secondary circuit fed by a primary circuit thru’ a heat
exchanger
 Reason to achieve the required accuracy on temperature control (55 °C ± 2°C)
FUEL (NG or Biogas) LOOPS
Priority is to draw/burn biogas off the digester
 IF NO gas OR
 NO flame (poor quality = unsuccessful (15 “sparking clicks”) = use FLARE!)
 TURN ON the NG boiler
 Repeat cycle (draw biogas) every 30 minutes
Water Loop
Water Loop
 The AD digester calls for heat using (temperature sensor T4) and switches on the
two circulating pumps.
 Temperature sensor T1 indicates temperature of the secondary circuit, and
should this reading fall below for example 70 oC then the biogas boiler is fired
up.
 If temperature sensor T2 does not make the required temperature in a define set
time (say 1 min.) then the natural gas boiler fires up in support, and its output is
modulated to deliver a set temperature at T3.
 An option is to use a mixing valve in the secondary circuit to limit AD feed-water
temperature for example to prevent digester overheating.
Fuel Loop
Fuel Loop
 Biogas from the AD digester is expected at 5 mbar, but can be boosted using a
fan to provide a high enough supply pressure to the modified biogas boiler.
 If there is no biogas take off and pressure limit reaches in the AD digester, the
pressure maintaining valve lifts to combust gas in the flare (which has a
support gas pilot light).
 If everything fails the Pressure Relieve Device (valve) releases gas to
atmosphere for safety
Water, biogas and natural gas loops
Water Loop
Biogas Loop
Natural gas Loop
Biogas inlet
 Biogas is piped from the biogas reactor via gas treatment to the biogas inlet on
boiler system. Biogas fed to the boiler must be clean and dry.
 The biogas feed splits: one way goes to the flare and the emergency blow off valve
and the other to the biogas solenoid.
Biogas
reactor
Gas
treatment
Boiler
system
Automatic functioning
• the solenoid is closed
IF
Not running
IF
Pressure rises above 35
mbar
IF
biogas pressure rising
above approx. 10 mbar
Heat is required
IF
the pressure of the reactor
and system drops
• biogas over 25 mbar is vented to the flare
via the vent spill regulator.
• emergency blow off valve releases biogas
pressure via the emergency biogas blow off
outlet
• the solenoid is opened
• the biogas blower starts raising biogas
pressure to 19 mbar to feed the biogas boiler
• the boiler is started
• the biogas boiler turns off
• the biogas blower stops
• the solenoid closes
Automatic functioning
Pressure sensor > 20 mbar?
 relay 1 high
 blower on
 biogas heat demand high (thermostat input)
 biogas solenoid open
 biogas boiler will start.
Pressure drops < 8 mbar?
 relay 1 low
 blower off
 biogas heat demand low
 biogas solenoid close
 biogas boiler will stop.
Automatic functioning
 The NG boiler is set to heat the primary water to a temperature below that of the
BB so the NG boiler should only fully fire when biogas is not available. The
BB temperature should be set slight higher than the NG boiler to allow it to use
available biogas in preference to using natural gas.
 If there is no biogas take off and the lower pressure limit reached in the AD
digester, then the support fuel will be called upon to provide the required heat.
Biogas boiler
 biogas available signal low (relay 1? adjustable value of setting)
 Boilers are set up to 82°C (relay 1? This value of setting is adjustable)
and will both reach and regulate to this
 Natural gas heat demand signal high or constant
 primary circuit pump on controlled by output from natural gas boiler
Natural gas
boiler
 Natural gas heat demand signal low
 Primary circuit pump off (controlled by natural gas boiler)
 Biogas boiler relay 1 inhibited
 Excess gas will vent to flare via flare spill valve.
Standard operation
Power on and E-stop in run position, the following will be in operation:
 Extractor fan on
 Biogas boiler power on constant
 Natural gas boiler power on constant
 Secondary circuit pump on constant
 24v power supply on constant
 Pressure sensor on constant (24v)
 PID* controller on constant (24v)
 3 way valve on constant (24v)
 Display gauges on constant (USB power supply)
*A proportional-integral-derivative controller (PID controller) is a control
loop feedback mechanism (controller) widely used in industrial control systems.
A PID controller calculates an error value as the difference between a measured process
variable and a desired setpoint. The controller attempts to minimize the error by
adjusting the process through use of a manipulated variable.
Settings
 The temperature in the primary is set by the individual boiler temperature
controls.
 The temperature setting for the bioreactor is set on the PID controller on the
front panel of the cabinet.
 The PID controller monitors the temperature in the bioreactor and controls a 3
way valve in the secondary circuit to supply sufficient heat from a heat exchanger
with the primary circuit. Over time this should achieve accurate control of the
temperature in the bioreactor.
Control panel information
RKC
CB100FK07-8*AN5N1
Control action
F
PID action with autotuning (reverse action)
Input range
K07
K type temperature sensor 0 to 1372 degree C
OUT1
8
Current 4 to 20mA
Alarm1/2
AN
Deviation high1
Comms
5
RS-485
Water proof
N
No
4-20mA out put on OUT2 contacts 3 and 4
The PID controller CB100FK07-8*AN5N1 has an RS 485 data output
The heat meter has a pulsed output.
No Alarm2
Security
 The safety system monitors for methane, CO, LPG (if used) and
fire and heat.
 If methane CO or LGP are detected the system shuts down and
leaves the extractor fan running.
 If fire or heat is detected the system including the extractor fan
shuts down and an external fire alarm output is available.
Alarms:
Methane or LPG or CO alarm ON
beacon sounder on
everything but extractor fan off
Heat / fire alarm ON
all off
External output
Power cycle to reset alarm?
Power off or E-stop off
all off
Connections
Connections into left side of boiler cabinet – dimensions to follow
Biogas in
Natural gas in
Biogas to flare
Biogas to emergency vent
Water flow
Water return
½” BSP female
½” BSP female
½” BSP male or compression
½” BSP female
35mm compression
35mm compression
Mains electricity
Fire alarm signal
1 phase mains electricity
Outputs out of back of unit
Connections
Biogas boiler exhaust
Natural gas boiler exhaust
Biogas boiler condensation drain
Natural gas boiler condensation drain
Primary water over pressure vent
Cabinet air ventilation outlet
Condensate drain 1 internal
Condensate drain 2 internal
Primary drain point internal
Secondary drain point internal
condensing boiler inlet and exhaust
condensing boiler inlet and exhaust
½” plastic
½” plastic
12mm copper
4” steel pipe
¾” BSP female
¾” BSP female
12mm compression
35mm compression
Installation
 The boiler unit must be installed in a weather proof container or building.
 Level and securely mounted to the wall of the container or building.
 Operating temperature between 0°C and 40°C.
 The exhausts of the boilers must pass through an air tight seal in the wall of the
container or building and vent outside at an appropriate position. This position
must be away from any window or door or ventilation intake as described in the





boiler flue installation instructions.
The ventilation fan exhaust must pass through an airtight seal in the wall of the
container or building to vent outside to an appropriate place.
The boiler condensate drains must discharge to an appropriate drainage point.
The blow off vent and the vent should be appropriately piped and located
according to SR25 zone 2 safe zone.
The boiler cabinet must be appropriately earthed to an earthing rod.
The container or building must have appropriate ventilation, to allow the unit
to ventilate itself.
Installation
 The boiler unit should be wired into the container or buildings gas and fire
alarm.
 The boiler unit should be wired to the container or buildings electricity supply
so that it is also isolated in the event of the container or buildings emergency
stop being operated or the electrical supply to the container or building being
isolated.
 The boiler cabinet must not be exposed to water or excessive moisture.
 The biogas condensate pots must be checked and drained of water daily.
 Need to check/service the combustion module every 6 months.
Thanks for the attention!
May you have any questions, please contact:
sandra.estevez@southwales.ac.uk
alex.chong@southwales.ac.uk